1. Field of the Invention
The present invention relates to an optical tomographic image obtaining apparatus that obtains optical tomographic images of targets of measurement, by irradiating the targets of measurement with measuring light beams, and employing interference between the measuring light beams, which are reflected at predetermined depths of the irradiated portions, and reference light beams. Particularly, the present invention relates to an optical tomographic image obtaining apparatus that comprises a focusing position moving means, for moving the focusing position of the measuring light beam.
2. Description of the Related Art
Development of optical tomographic image obtaining apparatuses, for obtaining optical tomographic images of living organisms and the like, is progressing. As the method of obtaining optical tomographic images, there are methods that employ optical interference by coherent light beams, to which frequency sweeping has been administered, and methods that employ optical interference by low coherence light beams.
OCT (Optical Coherence Tomography) apparatuses that obtain optical tomographic images, by measuring light intensities of low coherence coherent light beams with heterodyne demodulation are being put into practical use.
An OCT apparatus divides a low coherence light beam, emitted from a light source, such as an SLD (Super Luminescent Diode) into a measuring light beam and a reference light beam. The frequency of either the measuring light beam or the reference light beam is shifted by a piezoelectric element or the like. The measuring light beam is irradiated onto a target of measurement. The measuring light beam which is reflected from the irradiated portion and the reference light beam are caused to interfere, and the light intensity of the coherent light beam resulting from the interference is measured by heterodyne demodulation, to obtain optical tomographic data. Data regarding a predetermined depth of the irradiated portion (hereinafter, referred to as “measurement point”), at which the optical path lengths of the reference light and the measuring light are matched, is obtained by slightly moving mirrors provided along the optical path of the reference light beam, thereby slightly changing the optical path length of the reference light beam. Alternatively, optical tomographic images of predetermined regions may be obtained by repeating measurements while slightly shifting the position, onto which the measuring light is irradiated (as disclosed in “Real-time in vivo imaging of human gastrointestinal ultrastructure by use of endoscopic optical coherence tomography with a novel efficient interferometer design” by Andrew M. Rollins and Rujchai Ung-arunyawee, OPTICS LETTERS, Vol. 24, No. 19, pp. 1358-1360, Oct. 1, 1999).
By utilizing such an OCT apparatus, it is possible to diagnose the degree of penetration in early stages of cancer and the like. Therefore, methods for obtaining optical tomographic images from within body cavities by guiding measuring light beams and reflected measuring light beams with OCT probes, which are insertable into forceps channels of endoscopes, are being developed (as disclosed in Japanese Unexamined Patent Publication No. 2002-200037). Japanese Unexamined Patent Publication No. 2002-200037 discloses an OCT apparatus comprising an OCT probe, constituted by: an optical fiber, for guiding a measuring light beam; and a mirror, for reflecting the measuring light beam 90°, provided at the distal end of the optical fiber. The OCT probe is inserted into a body cavity via a forceps channel of an endoscope, and optical tomographic images of the walls of the body cavity are displayed by rotating the mirror at the distal end of the optical fiber.
Recently, resolution of images is becoming higher, due to improvements in light sources and the like, which are employed in OCT apparatuses. Resolution in the direction along the optical axis of a measuring light beam, that is, in the depth direction, has improved to the order of several microns. Meanwhile, it is necessary to decrease the beam diameter of the measuring light beam at the measurement point, in order to improve resolution in the direction perpendicular to the optical axis (hereinafter, referred to as “horizontal resolution”). An objective lens system having a great numerical aperture is required to decrease the beam diameter at the measurement point. However, in the case that an objective lens system having a great numerical aperture is employed, the focal depth becomes shallow. Accordingly, if the position of the measurement point in the direction of the optical axis shifts even slightly from the focal point, the horizontal resolution deteriorates conspicuously. For this reason, ACT apparatuses that have functions of moving the focal point positions of objective lens systems have been proposed (as disclosed in Japanese Unexamined Patent Publication No. 2003-199701).
In the optical tomographic image obtaining apparatus disclosed in Japanese Unexamined Patent Publication No. 2003-199701, a measuring light beam is irradiated onto a point of a target of measurement. The measurement point is moved along the direction of the optical axis at the irradiated portion, to obtain tomographic data up to a desired depth. When measurements are completed at that point, the irradiation position of the measuring light beam is moved slightly in a direction substantially perpendicular to the optical axis direction, and tomographic data is obtained up to a desired depth in a similar manner. These operations are repeated while slightly moving the irradiation position of the measuring light beam, to obtain optical tomographic images. Accordingly, it is necessary to move the focusing position of the measuring light beam to match the movement of the measurement point in the optical axis direction. Movement of the measurement point in the optical axis direction is performed at extremely high speed. Therefore, it is necessary for the movement of the focusing position of the measuring light beam to be performed at extremely high speed. However, a large and complex focusing position moving means is necessary in order to move the focusing position of the measuring light beam at high speed. This leads to a problem that optical tomographic image obtaining apparatuses become large and complex.